“It takes just two shots of the MMR vaccine to protect a child against measles, mumps and rubella for life. The same is true for polio and hepatitis B, a few injections grant life-long immunity against these viral diseases. By showing samples of the viruses to our immune system, we teach it to store a permanent memory of these enemies and guard against them in perpetuity.

Influenza is a different matter. There is a vaccine, but we have to take it every year. That’s because flu viruses evolve at tremendous speed. They copy themselves with surprising sloppiness, producing thousands of slightly different daughter viruses. If different strains infect the same cell, they can carry out the viral version of sex by mingling their genetic material to make hybrid daughters. And occasionally, entirely new strains that we’ve never encountered before can spill over into humans from animals.”